The billing meter of a color multi-function peripheral (MFP) charges different per-page rates for black and white and color copies. The MFP generally decides based upon whether the original is color or black and white. A black-white detection algorithm typically uses a color feature recognition to make the classification, such as non-neutral color, color edge, etc. Usually, this detection occurs in device-independent color space on the continuous tone color scan image.
In some instances, the system only has the binary halftone bitmap of the image available as the input to the black-white detection algorithm. This may occur because the processor in the image path does not have the black-white detection process built in to it. The system may also lack sufficient memory space to save the continuous-tone image. Determining the color or black and white status of a page becomes more difficult in the binary halftone image.
This difficulty may result from the binary bitmap including a mixture of CMYK (cyan magenta yellow black), even for an original page perceived as black and white page at viewing distance. For example, the scan image may not have exact color neutrality, even after scanner color calibration, because the scanner color response varies for the originals from different print technologies. Additionally, the printer engine may mix color (CMY) dots with black dots to enhance contrast and black and white text quality. The resulting issue lies in the use of color dots to render the near-neutral colors when the image converts to CMYK space from the scanner RGB space.
Another difficulty may arise from the device dependency of binary halftone images. Generally, no simply analytic transform exists to convert the binary halftone images back to device-independent color space, such as L*a*b, or other standardized color space. The conversion may need a four-dimensional look up table (LUT) for high color accuracy, adding significant memory overhead.
In addition, the conversion process may require several four-dimensional LUTs. For example, the device may use different conversions from L*a*b or other device-independent color space to the halftone binary image. Text mode may use one table, photo mode another table, with each mode requiring a different table. In some cases, the system may not have the capability to access the right LUT because of inaccessibility of the copy output intent information. Further, the bit depth decimation from 8 bit separation to 1 bit separation causes a significant loss of image information.
All of these factors combine to raise the difficulty of robust detection of a black and white page in the binary domain.